In most electrical alternating current machines, impedance is balanced. Thus, machine impedance is equal for different phases. However, machine impedance may be unbalanced in some special electrical machines.
For example, in order to achieve redundancy especially for off-shore wind turbines, generators with multiple three-phase winding are used for big wind turbines. Each set of three-phase winding may have a separately controlled frequency converter. With this kind of set up, a wind turbine can still produce electricity as long as at least one set of winding and its converter is in a good condition, even if all other windings and converters are broken. For a machine with multiple three-phase windings, mutual inductance between windings may change after one winding or its converter is broken. This causes an unbalanced impedance from the remaining windings. An unbalanced impedance may cause an unbalanced current or voltage at a machine stator and/or a converter. Due to unbalanced voltage and/or current, the stator power of the generator for example may have a harmonic at two times of the electrical frequency of the generator. As normally a bandwidth of a DC link voltage control loop is lower than this frequency, the balance of a grid power and the generator power cannot be achieved in such a situation. The harmonic in the stator power may cause a harmonic voltage at a same frequency at DC link capacitors of the frequency converter. The DC link voltage waveform in such a situation may comprise unwanted harmonics, which are not preferred, because a high harmonic at the DC link voltage may cause an over-modulation of the frequency converter and eventually trips the frequency converter.
Hence, there is a need for driving an electrical machine with an unbalanced impedance.